Spineless Wonders II: The Pelagic Fauna

What forms of life drift in the world's oceans? For centuries, we have
tackled this question by blindly pulling nets through the water, collecting
primarily the smaller, slower, more numerous, and more robust planktonic
species. Presently, we know a great deal about the more numerous crustacean
zooplankton, such as the copepods (the sea's insects) and euphausiids (krill),
that live in the upper layers of the ocean, but little about deep-water
and fragile life-forms (Image 1). Undoubtedly, we have adequately sampled
only a small fraction of the diversity present, resulting in significant
gaps in understanding the linkages between algal production and the top
predators in the oceans.

Undersea vehicles have vastly expanded our knowledge about
behavior, biodiversity and vertical distribution of pelagic animals. Remotely
Operated Vehicle (ROV's) and submersibles offer
opportunities for direct observations of the species that are normally
missed by plankton nets because they are too rare, too fragile, too deep,
or too fast for us to catch. In addition, submersible tools can capture
live, undamaged specimens of soft-bodied or gelatinous animals
for further observation or experimental study.

During this expedition, we will search the water column under
the Arctic ice-sheet for rare and new species of zooplankton using a variety
of techniques: Scuba divers, an ROV, and a wide variety of classical and
electronically controlled plankton nets collecting at great depth. We will
create image libraries of living animals, both known and unknown to science,
record their behavior and determine their molecular fingerprints [link
to barcode of life http://www.barcodinglife.com/]. The vertical distribution
and abundance of this fauna will be related to the physical structure of
the water column and that of their potential predators and prey.

Pelagic Fauna in the Arctic

Gelatinous zooplankton range from the weird to the wonderful. Most are fragile
and range from completely translucent to vividly pigmented. Individuals range
in size from only few millimeters to several meters in length, while colonial
forms may string tens of meters in deep-water. These animals are ubiquitous
in the oceans, and many species have persisted for hundred of millions of years,
yet we know relatively little about them. Collecting these animals with nets
usually destroys or breaks them into fragments that are then ignored, discarded,
or misidentified. Equally problematic, conventional preservatives often liquefy
them, leaving behind no identifiable remains.

The
hydromedusea Plotocnide borealis indicates the small end of the
size continuim for artic jellies. Click image for larger view.

In contrast, deep-water zooplankton are poorly known
not only because many are gelatinous, but because they require more
specialized electronic plankton nets, and take much longer to collect.
For example, collecting with inexpensive traditional plankton nets
to 100m might only take 10-20 minutes, while collecting to 3000m requires
more than 3 hours, making you unpopular with shipmates wanting to get
to the next station! Such deep-working
multiple-net systems divide the water column into pre-determined
layers, but the technology involved makes them worth more than the
typical automobile, and too pricy for many scientists.

Therefore, it is not surprising that scientists underestimate
the basic biodiversity as well as the abundance and biomass of both
gelatinous and deep-water animals, especially in Arctic seas where
sampling has been limited compared to other parts of the ocean. Historically,
most scientists assumed that these soft-bodied zooplankton were unimportant
to ecosystem functions, yet recent investigations have demonstrated
that they are capable of much higher rates of ingestion, growth, and
reproduction than crustaceans. Higher rates allow them to respond more
rapidly to shifts in ecosystem productivity. Deep-water species may
be present only in low numbers, but when summed over the wide depth
ranges in which they live, their importance becomes significant.

On our last expedition
in 2002 we had a number of surprises. Both the ROV and Scuba divers observed
a pronounced layer
of fragile ctenophores in shallow waters. ROV observations revealed that
larger zooplankton were broadly distributed throughout the water column, often
at comparable abundances to other oceans. A probable new species of larvacean
was observed below 2000 m. Even plankton nets revealed a surprisingly high biomass
of chaetognaths (arrow-worms) in the upper 100 m compared to other groups.
On the 2002 cruise, limited sampling opportunities with the ROV, plus limited
depth range on the plankton nets, constrained our efforts to find new species,
although we do have new records for existing species in the central basins
(see
Table below). With more extensive sampling efforts this expedition, we
are poised for greater discoveries.

Phyla

Group

World diversity

Total Arctic

Central Arctic

OE 2002

Cnidarians

hydromedusae

650

156*

56*

5*

siphophores

190

8

5

3

scyphozoans

150

7

1

3

Ctenophores

80

6

0

3

Nemertines

97

0

Annelids

polychaets

120

2

0

0

Mollusks

heteropods

35

0

0

0

pteropods

160

3

0

2

cephalopods

370

8

6

0

Crustaceans

cladocerans

8

4

0

1

ostracods

169

9?

8?

1

copepods

2000

156

97

>30

mysids

700

33

13

0

amphipods

400

10

8

4

euphausiids

86

7

3

1

Decapods

?

1

1

1

Chaetognaths

80

13

10

2

Tunicates

appendicularian/larvacean

64

5

0

3

pyrosomes

8

0

0

0

dolioids

17

0

0

1

salps

45

0

0

0

Fun Facts

Gelatinous zooplankton have only 1-10% of the carbon content (per gram wet-weight)
that we typically find in crustacean zooplankton.

Fishing nets were first developed by the siphonophore jelly-fish.
In the largest genus, Praya, their tentacular nets may be a meter tall, and
stretched along a rope-like colony that is 30-40 meters long.

Larvaceans live within large mucus structures called houses that
they build to help concentrate their food. Up to 20 such structures may
be built a day.

At peak burst speed, some copepods move 800 body-lengths per second. Other
groups may exceed this rate.

The original stealth technology was invented
by ctenophores, which use hundreds of tiny paddles of fused cilia to
glide through the water.

The largest members of many zooplankton groups
are found in cold or deep waters.

Zooplankton live longer in the Arctic. Groups
that typically only live a few weeks in the tropics may live several
years in the Arctic.

Deep-water zooplankton are often more brightly
colored that shallow water relatives because bright colors cannot
be seen at depth by predators.